1. Field of the Invention
The present invention relates to an angle sensor which detects, for example, the steering angle of a steering wheel of an automobile, and, more particularly, to an angle sensor which can detect the angle of rotation of the steering wheel with high precision as a result of preventing rattling caused by backlash between gears or between a gear and a screw shaft inside the angle sensor.
2. Description of the Related Art
FIG. 8 is a plan view showing the internal structure of a conventional angle sensor proposed in U.S. patent application Ser. No. 09-477971 by the inventor of the angle sensor in the application concerned. The conventional angle sensor is used to detect, for example, the steering angle of a steering wheel of an automobile with high precision.
In an angle sensor 1 shown in FIG. 8, a rotary member 3 is provided inside a case 2 formed of a synthetic resin material such as plastic. The rotary member 3 is a cylindrically shaped member formed of, for example, a synthetic resin material, and is rotatably supported with respect to the case 2. The steering wheel of an automobile is inserted into the rotary member 3, and the rotary member 3 rotates clockwise and counterclockwise along with the steering wheel. A plurality of helical gears 3a are formed along the entire outer peripheral surface of the rotary member 3.
A rotary shaft 9 is rotatably provided inside the case 2, with the illustrated X dimension being defined as the center axis thereof. A driving gear 8 is secured to the rotary shaft 9. A plurality of helical gears 8a are formed along the entire outer peripheral surface of the driving gear 8, and engage the helical gears 3a of the rotary member 3. The rotary shaft 9 is formed of a metallic material such as brass or aluminum, and has a spiral thread groove 9a formed from the center to one end thereof. A detecting member 4 is provided at the rotary shaft 9.
The detecting member 4 has a through hole formed therein from one end surface to the other end surface thereof in the direction of movement (that is, in the X dimension). Threads (not shown) which engage the thread groove 9a formed in the rotary shaft 9 are formed in the inner peripheral surface defining the through hole. A second magnet 5B is mounted to the bottom surface of the detecting member 4 by, for example, insert molding. The detecting member 4 is guided in the interior of the case 2 so as to move in a straight line in the X dimension. When the rotary member 3 rotates, the driving gear 8 and the rotary shaft 9 rotate, causing the detecting member 4 and the second magnet 5B to reciprocate in the X dimension.
A Hall element 6B is provided at a side opposing the second magnet 5B, on a fixing member 7 in the case 2. A magnetized first magnet 5A is integrally mounted to the outer circumferential surface of the rotary shaft 9. A second Hall element 6A is provided on the fixing member 7 so as to oppose the outer circumferential surface of the first magnet 5A.
When the rotary shaft 9 rotates with the rotation of the rotary member 3, the first magnet 5A rotates, causing a sine wave to be output. At the same time, the second magnet 5B reciprocates in the X dimension, causing values which change linearly through the entire rotational angle range of the steering wheel to be output. By detecting these values, the absolute position of the steering wheel resulting from the angle of rotation (that is, the steering angle) can be detected.
When an attempt is made to detect the angle of rotation (that is, the steering angle) of the steering wheel with high precision using the above-described angle sensor 1, the problem of rattling due to backlash between the helical gears 3a of the rotary member 3 and the helical gears 8a of the driving gear 8 arises.
More specifically, in the above-described angle sensor 1, the rotation of the rotary member 3 is detected, or the detecting member 4 is moved in the X dimension after changing the rotational movement to linear movement. Therefore, when the backlash between the helical gears 3a of the rotary member 3 and the helical gears 8a of the driving gear 8 is too large, the rattling therebetween causes an error to be produced in the-distance of movement of the detecting member 4 which is moved in the X dimension. Here, when the direction of rotation of the rotary member 3 (or the steering wheel) changes, the driving of the driving gear 8 is reduced in correspondence with the amount of rattling caused by the backlash, causing hysteresis resulting from the direction of rotation to occur, whereby the amount of error is increased.
In the conventional angle sensor 1, backlash which occurs between the gears is decreased by increasing the degree with which the helical gears 3a of the rotary member 3 and the helical gears 8a of the driving gear 8 contact each other by biasing, for example, either the rotary member 3 in the Z2 direction towards the driving gear 8 or the driving gear 8 in the Z1 direction towards the rotary member 3.
However, when the degree with which the rotary member 3 and the driving gear 8 contact each other is increased to the extent that rattling does not occur, the biasing force produced between the gears becomes too large, so that rotational torque becomes large, and the rotary shaft 9 gets distorted due to, for example, thermal expansion, making it impossible to smoothly guide the detecting member 4 in the X dimension. In addition, the rotary shaft 9 has a simple beam structure in which both ends thereof are supported, so that, when a biasing force acts, the displacement of the portion corresponding to where the rotary member 3 and the driving gear 8 engage (that is, the center portion) is displaced the greatest, with the displacement becoming smaller towards both ends of the rotary shaft 9. Therefore, the rate of change of the strength of the magnetic field in the Z dimension with respect to the distance of movement of the second magnet 5B and the detecting member 4 is no longer linear with respect to the direction of movement. In addition, the distance between the first magnet 5A and the first Hall element 6A changes so that a predetermined output can no longer be obtained. Therefore, the distance of movement of the detecting member 4 can no longer be detected precisely, so that the precision with which the angle of rotation (that is, the steering angle) of the rotary member 3 (or the steering wheel) is detected is decreased.
In the above-described conventional angle sensor 1, the detecting member 4 and the screw shaft 9a are constructed so that only the screw shaft 9a engages the threads formed in the through hole 4a of the detecting hole 4. Therefore, play (that is, backlash) tends to occur between the threads of the detecting member 4 and the thread grooves of the thread groove 9a. The backlash tends to result in rattling of the detecting member 4 in the axial direction (that is, the X dimension), so that a precise distance of movement in accordance with the angle of rotation of the rotary member 3 cannot be obtained by the detecting member 4. Therefore, the Hall element 6B can no longer be used to detect with high precision the angle of rotation of a shaft to be detected, such as a steering wheel.
To overcome the aforementioned conventional problems, it is an object of the present invention to provide an angle sensor which can detect the angle of rotation of a rotary member with high precision as a result of decreasing backlash between a driving gear and a gear of the rotary member.
It is also another object of the present invention to provide an angle sensor in which a detecting member which reciprocates in accordance with the rotation of a screw shaft is advanced with high precision.
To these ends, according to a first aspect of the present invention, there is provided an angle sensor for detecting an angle of rotation of a first rotary shaft by a detecting operation of a detecting portion. The angle sensor comprises a first gear which rotates in accordance with the first rotary shaft, a second rotary shaft which extends in a direction perpendicular to the first rotary shaft, a second gear which rotates along with the second rotary shaft and which engages the first gear, a third gear which is rotatably provided at the second rotary shaft and which engages the first gear, and the detecting portion which detects the rotation of the second rotary shaft. In the angle sensor, the first gear engages the second gear and the third gear in a screw gear relationship, and a biasing member is provided at the second rotary shaft. The biasing member causes a tooth of the first gear to be sandwiched between a tooth of the second gear and a tooth of the third gear.
In one form of the first aspect of the invention, the biasing member may exert a biasing force onto the second gear in a direction of rotation thereof.
It another form of the first aspect of the invention, the biasing member may exert a biasing force in a direction in which the second gear approaches the first gear.
In the invention, the second gear (that is, a driving gear) and the third gear (that is, an auxiliary gear) are disposed at the first gear (that is, a rotary member) through the biasing member, so that a tooth of the second gear as well as a tooth of the third gear resiliently presses against a tooth of the first gear. Here, the second rotary shaft rotates similarly to the second gear.
Accordingly, it is possible for the second gear to always drive the second rotary shaft while the second gear resiliently presses against the first gear at all times. This makes it possible to eliminate rattling caused by backlash between the first and second gears. Consequently, the rotational force which is applied to the rotary member from the steering wheel can be transmitted to the driving gear with high efficiency, thereby allowing the angle of rotation of the steering wheel to be detected with high precision.
In still another form of the first aspect of the invention, the first gear and the second gear may engage each other at an intersection point where an imaginary normal line from the center of rotation of the first gear vertically intersects the second rotary shaft, and the third gear may be provided at a location situated away from the intersection point.
By virtue of this structure, the rotational force of the first gear can be transmitted to the second gear with precision and with high efficiency.
When the first gear and the second gear engage each other at an intersection point where an imaginary normal line from the center of rotation of the first gear vertically intersects the second rotary shaft, and when the third gear is provided at a location situated away from the intersection point, the third gear may be a helical gear having an inverted spherical surface.
By virtue of this structure, the first and second gears can be disposed in a row with the second rotary shaft, so that they can be disposed in a smaller area.
According to a second aspect of the present invention, there is provided an angle sensor for detecting an angle of rotation of a first rotary shaft as a result of a detecting operation by a detecting member. The angle sensor comprises a first gear which rotates in accordance with the first rotary shaft, a screw shaft which extends in a direction perpendicular to the first rotary shaft, a second gear which rotates with the screw shaft and which engages the first gear, a fitting member which engages the screw shaft and which moves in an axial direction of the screw shaft by a rotational force of the screw shaft, a detection member to be detected which moves along with the fitting member, a detection portion to be detected provided at the detection member, and a detecting member for detecting a linear movement of the detection portion. In the angle sensor, the fitting member and the detection member are connected together by a plate spring, and the fitting member is supported by the plate spring. In addition, a plate thickness direction of the plate spring is oriented in a direction perpendicular to a direction of movement of the fitting member and the detection member. Further, the plate spring is secured to the fitting member and the detection member in the direction perpendicular to the direction of movement of the fitting member and the detection member and at a location where a gap is formed in a plate surface direction. The plate spring is secured along a line facing the direction of movement of the fitting member and the detection member.
In the invention, the plate thickness direction (that is, a surface direction) of the plate spring faces the resiliency dimension (that is, the Z dimension) of the plate spring, and the fitting member and the detection member are secured along a line which extends in the resiliency dimension of the plate spring. This prevents the plate spring from becoming twisted, thereby allowing the fitting member to be stably fitted to the screw shaft, so that the detection member can be reliably moved.
In one form of the second aspect of the invention, a mounting surface of the plate spring for mounting to the fitting member and a mounting surface of the plate spring for mounting to the detection member may be located in the same plane.
By virtue of this structure, the angle sensor can be made thinner.
In another form of the second aspect of the invention, the fitting member may comprise a U-shaped fitting portion which opens in a direction perpendicular to a plane of the plate spring, the fitting portion engaging the screw shaft.
In the above-described structure, an opening can be formed in a portion of the fitting portion by forming the fitting portion into a U shape in cross section. Therefore, it is possible to easily perform mounting and dismounting operations of the screw shaft and the fitting portion through this opening.
When the fitting member comprises a U-shaped fitting portion which opens in a direction perpendicular to a plane of the plate spring, and the fitting portion engages the screw shaft, the fitting member may comprise a pair of the fitting portions which are separated from each other in the direction of movement thereof. In addition, the plate spring may have a cutaway portion formed in the center portion thereof, and the pair of fitting portions may be biased towards the screw shaft by an area of the plate spring where the cutaway portion is not formed.
By virtue of this structure, the pair of fitting portions can be independently pushed against the screw shaft, so that they can stably press against the screw shaft without tilting.
In still another form of the second aspect of the invention, the angle sensor may further comprise a guiding member for guiding the movement of the detection member in an axial direction thereof. The guiding member has at least one rail which is provided parallel to the screw shaft. The detection member slides on the at least one rail.
In other words, one rail or two or more rails may be used. When a rail or rails which are parallel to the screw shaft are provided, the detection member can move smoothly in a straight line with respect to a detecting means. (that is, a Hall element).
Two of the rails parallel to each other may be provided along the direction of movement, and the fitting member may be located substantially at the center of a region between the two rails.
By virtue of this structure, the load of the holder which holds the detecting member can be distributed uniformly on the two rails, so that it is possible to prevent problems such as tilting of the holder and derailment of a slider of the holder from the rails from occurring. Therefore, it is possible to always move the detection member parallel to the detecting member, thereby allowing very small angles of the first rotary shaft which is a detect shaft (that is, a steering wheel) to be stably detected.